I agree with you that JGL is a fantastic actor - it seems strange to me that he gets handed two-dimensional characters in Nolan films (Inception, TDKR).

I don't really think Arthur from Inception is comparable to Blake.

Arthur, and none of the team were characters (to the point that some people theorize that they are a part of Leo's subconscious...). Which led me to be confused when critics were praising JGL's performance. What performance? Who was Arthur again?

I didn't get that same experience with Blake. Blake was written fine (at least not in the same category as Arthur), and JGl was great in the role.